The relative effects of specimen thickness and convergence on the weakbeam contrast of stacking faults

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Abstract
It has long been held that the average weak-beam TEM contrast of an intrinsic fault with a localized displacement should not change on reversing the sense of the reflection used. It is demonstrated that, if the averaging procedure is carried out appropriately, this is not the case on either kinematical or dynamical theory of diffraction contrast. A re-analysis of dynamical theory is presented to demonstrate the significance of the beam convergence used in both the collection of experimental data and in the averaging of theoretical simulations. A quantitative assessment of experimental data for extrinsic faults in GaAs and extrinsic and intrinsic faults in Ni3 Al is presented and discussed in the light of established theory, with the conclusion that experimental asymmetries are typically rather larger than expected, perhaps because of inadequate treatment of contributions to the contrast by inelastically scattered electrons.